The present invention relates to a sorting method of monolithic ceramic capacitors. It is especially related to the improvement for achieving the efficiency of the sorting process, and the improvement in the reliability of the sorting result.
When a foreign substance is mixed or an aggregate is generated in a ceramic dielectric of the monolithic ceramic capacitor during its manufacturing process, such a problem that void regions are caused in the ceramic dielectric after baking may be encountered. Because this defect leads to a degradation of the insulation resistance of the monolithic ceramic capacitor, at a stage before shipment at least, the product having such a defect must be sorted and removed.
In order to sort and remove such defective products, measuring the insulation resistance of the monolithic ceramic capacitors, with providing predetermined conditions in the manufacturing process thereof, is usually performed.
However, the method of measuring the above-mentioned insulation resistance and judging the existence of defect products tends to detect the existence of defect products only indirectly. Therefore, when the defect is on the micro scale, this may be unable to be detected by the indirect method using the evaluation of insulation resistance. In addition, in a certain meaning, it is undetectable by the evaluation of insulation resistance, in other words, the defects on the micro scale with the insulation resistance appearing normal, it may not pose a problem practically either, even when it exists.
However, if thin layering of the dielectric of the monolithic ceramic capacitor progresses, even when it is the micro defect which does not pose a problem practically as mentioned above, the insulation resistance will degrade in long-term usage, and failure by the defect may result. Therefore, implementation of the sorting method whereby a defect is detected with higher reliability is desired as thin layering of dielectric progresses in this way.
Currently, there are many applications of the monolithic ceramic capacitors in military, space, motor vehicle, and the like. In the monolithic ceramic capacitors for these applications, high reliability in sorting result is required. As a method whereby removal of inferior goods and confirmation of quality can be made with high reliability in relation to this, the following U.S. military specification is known.
(1) MIL-STD 39014 item 4.72 xe2x80x9cVoltage conditioningxe2x80x9d xe2x80x9c. . . The double voltage as the rating voltage defined at the maximum allowable working temperature of the capacitor is applied for 96 hours, thereby a defect is made to appear. The subsequent insulation resistance is measured at a normal temperature, and a defect is detected by the degradation of this insulation resistance.xe2x80x9d This is one of the burn-in process.
(2) MIL-STD 55681C item 3.8 xe2x80x9cInsulation-resistance b. at 125 degrees C.xe2x80x9d xe2x80x9c. . . at 125 degrees C., insulation resistance is measured at the rated voltage to confirm that it is a resistance value more than the standard value.xe2x80x9d
However, there is a problem which should be solved in these defect-detection methods by the U.S. military specification mentioned above.
First, because the conditions exerted on a capacitor are severe according to the method (1) of the above, a reliable evaluation result is obtained. However, in order to obtain such an evaluation result, it requires at least 96 hours, and it is not efficient. Therefore, when all of many monolithic ceramic capacitors must be evaluated, it is not practical.
Next, according to the method (2) of the above, an evaluation can be completed in a comparatively short time. However, about the reliability of the evaluation result, it is not satisfied compared with the method (1).
Consequently, the objective of this invention is to provide an efficient sorting method of the monolithic ceramic capacitors with reliable sorting result.
The sorting method of the monolithic ceramic capacitors based on this invention has a characteristic at the burn-in process for making appear the defect which is inherent in the monolithic ceramic capacitor to be sorted. In order to solve the above-mentioned technical problem, at the temperature of 70-140 degrees C., a voltage is applied to the monolithic ceramic capacitor so that the electric field strength applied between the internal electrodes opposing each other may be 7-30 kV/mm. The insulation-resistance measurement process which measures the insulation resistance of the monolithic ceramic capacitor after performing a burn-in process. And, it is made to perform the sorting process which sorts the monolithic ceramic capacitor based on the insulation resistance measured during this insulation-resistance measurement process. A large voltage acceleration property is obtained by providing the above conditions during the burn-in process, and thus reducing the time of sorting.
This invention is especially applied advantageously to the monolithic ceramic capacitor whose thickness of the dielectric layer positioned between the internal electrodes opposing each other is 5 micrometers or less.
Moreover, in this invention, it is sufficient if the burn-in process is performed just for 2-300 seconds.
Moreover, in this invention, the insulation-resistance measurement process is performed, preferably applying the rated voltage of a monolithic ceramic capacitors at a normal temperature.